The present invention relates to motor controls and, more particularly, to control systems that prevent heat from damaging vehicle power components, such as motor and power heads, during their operation.
The reliability of electric DC motors and their power amplifier circuits is greatly reduced by thermal stresses. Typically these devices are protected by thermal switches which shut off power to the control system during over-temperature conditions. This is often unacceptable in industrial truck applications since the operator is left with a inoperable vehicle.
Typically, the thermal switch shut-off temperature is selected to be near the system's thermal limit so as to maximize productivity. However, many times the damage is already done before the thermal switch is activated.
U.S. Pat. No. 4,626,753, issued to Letterman, teaches a process for controlling the speed of an electric motor. Motor temperature is sensed and compared to a reference value. The two values are summed and employed as a reference of motor current. Motor current is sensed and compared to the reference value and a prescribed setpoint. The three values are summed and employed as a reference of motor speed. When actual motor temperature exceeds a given value, the current provided to the motor thereby reducing the speed thereof is reduced. This in turn reduces the I.sup.2 R power loss manifested as heat within the motor, thus eventually decreasing the temperature of the motor.
Another common method of controlling motor damage due to thermal stress is to sense the actual temperature, through use of a thermistor, and then reduce the motor speed as a function of the measured temperature. In this way, the operator is forewarned of the overheating condition and can take the appropriate steps to reduce the work load on the vehicle. Should the operator ignore or fail to recognize the warning, however, the motor will eventually overheat and probably be damaged. The power amplifier is also at risk of damage in this situation.
Unfortunately, merely reducing motor speed does not always guarantee that the heat producing currents of the motor and power amplifier will be reduced. Therefore, to effectively reduce these currents and consequently allow the motor and power amplifier to cool, down, it becomes necessary to limit the velocity and acceleration (rate of change of velocity) of the vehicle.
The current drawn by a DC motor is controlled by the required torque. The torque is a function of friction, velocity, and acceleration, as represented by the following equation: EQU T=K.multidot..phi..multidot.i=J.multidot..alpha.+B.multidot..omega.+F
where:
T=torque of the motor, N.m PA1 J=inertia of the truck reflected back on the motor, Kg.multidot.w.sup.2 PA1 .omega.=angular velocity of the motor, rad/sec PA1 B=viscous damping of the truck reflected back on the motor, (N.m)/(rad/sec) PA1 .alpha.=angular acceleration of the motor, radians PA1 F=constant friction of the truck reflected back on the motor, N.m PA1 K=torque constant of the motor, N.m/webers.amps PA1 .phi.=air gap magnetic flux of the motor, webers PA1 i=armature current of the motor, amps
Since truck inertia J is typically much larger than viscous damping B, the torque required to accelerate the vehicle is greater than the torque required to maintain it at a given speed. For applications that routinely require speed variations, such as material handling vehicles, the term of the above equation relating to acceleration (J.multidot..alpha.) becomes a much more significant contributing factor to the overall torque equation than it does in a "steady state" (i.e., constant velocity) situation. impact on motor current than does velocity.
Therefore, the invention departs from the common, prior art teaching of speed reduction as a means of controlling motor overheating. Rather, this invention concentrates upon reducing acceleration, as the primary technique of relieving the overheated condition.
The present invention provides the operator with feedback information to control the vehicle before shutdown occurs. The truck operator is given the option of using several levels of derated performance in which to effectuate vehicle cool down. A truck equipped with circuitry for implementing this inventive performance limiting scheme is protected from experiencing a shutdown due to excessive temperature.
The temperature limiting technique of this invention introduces hysteresis between each performance limiting step. This means that the truck must be allowed to cool down a predetermined number of degrees below the trip point, before performance can be restored a single incremental step. This procedure eliminates cyclic surging that would normally occur at the limit trip point absent hysteresis.
It is also possible in accordance with this invention, to create a continuous and substantially linear function of vehicular performance. In such a technique the truck performance is limited as a function of temperature. Since the performance of the vehicle is defined by both velocity and acceleration, it follows that for a continuous technique, the incremental performance levels become too small to require intermediate hystere is adjustments.